Solar energy is energy from the sun that is converted into thermal or electrical energy.

Solar power is the generation of electricity from sunlight. This can be direct as with photovoltaics (PV), or indirect as with concentrating solar power (CSP), where the sun's energy is focused to boil water which is then used to provide power.

Google Clean Energy 2030 advocates that solar, both PV and CSP, could grow from about 1 GW in 2009 to 250 GW by 2030, generating 12% of U.S. electricity demand.

Development, deployment, and policy

1800s-early 1960s

In the 1890s, solar water heaters were being used all over the United States. At the time, they were cheaper than wood and coal-burning stoves: artificial gas made from coal was available to heat water, but cost 10 times the price paid for natural gas in 2010, while electricity was even more expensive. Instead, many homes used solar water heaters. In 1897, 30 percent of the homes in Pasadena, CA, were equipped with solar water heaters. As mechanical improvements were made, solar systems were used in Arizona, Florida, and other states of the U.S.[1]

By 1920, ten of thousands of solar water heaters had been sold. By then, however, large deposits of oil and natural gas were discovered in the western United States. As fossil fuels became increasingly available, solar water systems began to be replaced with heaters burning fossil fuels. Solar water heaters, however, are making a comeback: there are more than half a million of them in California in 2010. They heat water for use inside homes and businesses, and can also heat swimming pools. Panels on the roof of a building can contain water pipes, and when the sun hits the panels and the pipes, the sunlight warms them, and that warmed water can then be used.[1]

1970s

The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now the [[renewable energy|National Renewable Energy Laboratory), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). Much of the U.S. support promoted by President Carter for renewable energy, including solar, was through tax credits.[2]

1980s-2000

Between 1970 and 1983 photovoltaic installations grew rapidly, but falling oil prices in the early 1980s moderated the growth of PV from 1984 to 1996. President Reagan also undid many of the renewable energy tax credits and research funding put in place by President Carter.[2]

2000-Present

Photovoltaic production has been doubling every 2 years, increasing by an average of 48 percent each year since 2002.[3] At the end of 2008, the cumulative global PV installations reached 15,200 megawatts.[4] Roughly 90% of this generating capacity consists of grid-tied electrical systems. Solar PV power stations have capacities ranging from 10-60 MW, although proposed solar PV power stations would have a capacity of 150 MW or more.[5] Since 2006, investors are increasingly installing photovoltaics for free in return for a long term power purchase agreement: 50% of commercial systems were installed in this manner in 2007.[6]

Commercial concentrating solar thermal power (CSP) plants were first developed in the 1980s, and is becoming more developed and widespread: the 11 MW PS10 power tower in Spain, completed in late 2005, is Europe's first commercial CSP system, and a total capacity of 300 MW is expected to be installed in the same area by 2013.[7] In 2010, Bloomberg New Energy Finance forecast that solar power may almost double in Germany that year, from 9,786 megawatts to 18,000 megawatts.[8]

In May 2011, Lowe's partnered with solar installer Sungevity to have in-store displays at some locations where customers can get a quote for installing solar panels based on satellite images, buy the panels directly, and pay for them through monthly leasing rather than paying the full amount upfront. The move is seen as part of a broader push to make solar more widely available and affordable.[9]

Solar thermal energy

Solar thermal energy is a technology for harnessing solar energy for thermal energy (heat). Solar thermal collectors are defined by the US Energy Information Administration as low-, medium-, or high-temperature collectors. Low temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for creating hot water for residential and commercial use. High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. STE is different from photovoltaics, which convert solar energy directly into electricity. While only 600 megawatts of solar thermal power is up and running worldwide in October 2009, according to Dr David Mills of Ausra, another 400 megawatts is under construction and there are 14,000 megawatts of the more serious concentrating solar thermal (CST) projects being developed.[10]

The most common usage of solar thermal energy is for on site water and space heating. However, with high temperature collectors, electrical energy has been reliably produced by concentrated solar power arrays: mirrors focusing light onto pipes of water or other heat transfer fluid. The hot water is heated to the boiling point and powers a steam turbine to generate electricity, or is preheated for use in fossil fuel based generation. [11]

Solar Cells or Photovoltaic Energy

Solar radiation can be directly converted to electricity using solar cells, also called photovoltaic (PV) cells (photo meaning "light" and voltaic meaning "electricity").[5] They were first developed in the 1950s for use on U.S. space satellites, and can be found on many small appliances, like calculators. Most are made of silicon, a special type of melted sand. They can be arranged in panels, like those found on rooftops.[1]

When sunlight strikes the solar cell, electrons are knocked loose. They move toward the treated front surface. An electron imbalance is created between the front and back. When the two surfaces are joined by a connector, like a wire, a current of electricity occurs between the negative and positive sides. These individual solar cells are arranged together in a PV module and the modules are grouped together in an array. Some of the arrays are set on special tracking devices to follow sunlight all day long. The electrical energy from solar cells can then be used directly, such as in a home for lights and appliances, or in a business. Solar energy can also be stored in batteries. Some experimental cars also use PV cells, and they convert sunlight directly into energy to power electric motors on the car.[1]

Concentrating solar power

Concentrated solar power (CSP) systems use lenses or mirrors to focus a large area of sunlight onto a small area. Electrical power is produced when the concentrated light is directed onto photovoltaic surfaces or used to heat a transfer fluid for a conventional power plant.[12]

Types

Concentrated solar power systems are divided into:

concentrated solar thermal (CST)

concentrated photovoltaics (CPV)

concentrating photovoltaics and thermal (CPT)

Concentrated solar thermal

Concentrated solar thermal (CST) is used to produce renewable heat or electricity (generally, in the latter case, through steam). CST systems use lenses or mirrors and tracking systems to focus a large area of sunlight onto a small area. The concentrated light is then used as heat or as a heat source for a conventional power plant (solar thermoelectricity).[13]

Some thermal solar power plants use a highly curved mirror called a parabolic trough to focus the sunlight on a pipe running down a central point above the curve of the mirror. The mirror focuses the sunlight to strike the pipe, and it gets so hot that it can boil water into steam. That steam can then be used to turn a turbine to make electricity. In California's Mojave desert, there are huge rows of solar mirrors arranged in what's called "solar thermal power plants," which makes electricity for more than 350,000 homes. Some solar plants, are a "hybrid" technology: during the daytime they use the sun, and at night and on cloudy days they burn natural gas to boil the water so they can continue to make electricity.[1]

Another form of solar power plants to make electricity is called a solar tower. Sunlight is reflected off mirrors circling a tall tower. The mirrors are called heliostats and move and turn to face the sun all day long. The light is reflected back to the top of the tower in the center of the circle where a fluid is turned very hot by the sun's rays. That fluid can be used to boil water to make steam to turn a turbine and a generator.[1]

Concentrated photovoltaic

Concentrated photovoltaic (CPV) systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties may be used, and these are often mounted on a solar tracker in order to keep the focal point upon the cell as the Sun moves across the sky.[5]

Concentrating Photovoltaics and Thermal

Concentrating Photovoltaics and Thermal (CPVT) technology produces both electricity and thermal heat in the same module. Thermal heat that can be employed for hot tap water, heating and heat-powered air conditioning (solar cooling), desalination or solar process heat.[14]

Photovoltaic Costs

In the article, "Shining Light on the Cost of Solar Energy" in National Geographic, estimates of the costs of photovoltaics (PV) vary because there are so many different factors to be taken into account, and no one established way for doing so. According to the article, a standard way of thinking about the cost of electricity is “total system levelized cost,” or how much a power producer would have to charge for electricity to earn back the money spent building a new generating facility. Yet there is no one accepted way to do a levelized cost calculation for solar PV. Figuring out the levelized cost requires a number of estimates, including ecological factors like sunshine, leading to different estimates.[15]

Most solar cells are made from silicon, the same semiconductor material used for computers. The cells are expensive to produce because it takes a great deal of energy to purify the silicon. And, while the computer industry has made enormous strides in making cheaper silicon devices, those advancements don’t translate to the solar industry, where the cost is based more on the actual silicon material, not the design. What may help to lower the cost of solar are new technologies like nanotechnology and new materials like cadmium telluride - it is cheaper to make “thin-film solar cells” with cadmium telluride than with silicon.[15]

But that still leaves “soft costs,” which includes everything from permitting fees to the hardware that mounts solar panels onto a roof. Even though there is disagreement over how much of the price of solar is tied up in these soft costs, they are important factors. Ryne Raffaelle, director of the National Center for Photovoltaics at the National Renewable Energy Laboratory, says the solar array itself accounts for only half the cost of a solar system today.[15]

According to New Energy Finance estimates, electricity from coal costs about 7 cents a kilowatt hour compared with 6 cents for natural gas and 22.3 cents for solar photovoltaic energy in the final quarter of 2010. But Canadian Solar said such comparisons often overstate the costs of solar because they may take into account the prices paid by consumers and small businesses who install roof-top power systems, instead of the rates utilities charge each other.[16] In Hawaii and some parts of the Southwest, solar has already dipped below “grid parity” - produced and sold for less than the average, going electric rate.[15]

October 2010: White House to install solar panels

In October 2010, the Obama Administration announced plans to install solar photovoltaic panels and a solar water heater on the roof of the White House by spring 2011. The first solar photovoltaic system on the White House grounds was installed quietly in 2002 on a maintenance building, during the administration of President George W. Bush. It continunes to produce electricity used in maintaining the White House grounds. A solar hot-water system put in place during the administration of President Jimmy Carter was later removed under President Ronald Reagan, ostensibly to permit roof repairs, and was never reinstalled. Many, however, saw it as symbolic of Reagan's opposition to renewable energy, as he slashed funding for many of Carter's clean energy programs.[17]

The climate change organization 350.org and the Solar Association have been working with the Obama administration to install solar energy systems on government buildings across the country. Along with the White House announcement, the Energy Department released a report titled "Procuring Solar Energy: A Guide for Federal Facility Decision Makers" to assist them in arranging for solar installations.[17]

The organization ICLEI Local Governments for Sustainability USA, representing more than 600 U.S. local governments that have committed to climate protection and sustainability, also praised the Obama administration's announcement. The group, whose membership includes small towns and large cities, from the Fairbanks North Star Borough in Alaska to the city of Miami, took the opportunity to call on the president and Congress to help restore tax-assessed financing programs for solar electricity and other energy improvements. Known as PACE, for Property Assessed Clean Energy, the financing programs were thrown into limbo in early 2010 because of opposition from the Federal Housing Finance Agency and the mortgage enterprises Fannie Mae and Freddie Mac. Patrick Hays, mayor of North Little Rock, Ark., and president of ICLEI USA's board of directors, said, "As mayors and county executives continue to battle persistently high unemployment, a fragile economy and the impact that climate change is having on our communities, Congress and the president must help local communities by allowing them to continue their PACE programs. The time to act on PACE is now."[17]